Computed tomography (CT) is an imaging diagnostic examination. This technique was once called computerized axial tomography. X-ray tomography is a three-dimensional radiological medical image reconstructed by digital geometric processing. This technology mainly irradiates the human body through single-axis X-ray rotation. Because different tissues have different absorption of X-rays, three-dimensional computer technology can be used to reconstruct tomographic images. By processing the window width and window level, the tomographic images of the corresponding tissues can be obtained. A stereoscopic image can be formed by stacking tomographic images layer by layer. X-ray tomography is a three-dimensional radiological medical image reconstructed by digital geometric processing. This technology mainly irradiates the human body through the rotation of X-rays in a single axial plane. Because different biological tissues have different absorption rates (or radiation densities) for X-rays, three-dimensional computer technology can be used to reconstruct tomographic images. After window value and window level processing, the relative gray image can be obtained. If the images are superimposed by computer software, a three-dimensional image can be formed. Advantages and dangers are better than X-rays. First of all, X-ray tomography provides doctors with complete three-dimensional information of organs, while X-ray images can only provide overlapping projections of multiple sections; Secondly, because of the high resolution of computed tomography, the difference of radiation density obtained by different tissues can be distinguished even if it is less than 65438 0%. Thirdly, because tomographic imaging technology provides three-dimensional images, we can see axial, coronal and sagittal images according to different diagnostic needs. We call it multiplane reconstruction imaging. In addition, the image of any section can be generated by interpolation technology. This has brought great convenience to diagnosis and scientific research. Radiation dose X-ray tomography is considered as a diagnostic technique of moderate to high radiation. Although technological progress has improved the radiation efficiency, at the same time, in order to improve the image quality or for more complex technology, there is still the consideration of increasing the dose. The evolving resolution enables computed tomography to carry out new research and has more advantages: for example, compared with traditional angiography, computed tomography can avoid inserting venous tubes and venous catheters; Computed tomography of the large intestine is also used to diagnose tumors, such as barium contrast examination of the large intestine, but the dose is low. Its convenience and applicability are getting stronger and stronger, and it is becoming more and more popular. Recently, in the comprehensive evaluation of Britain, computed tomography accounted for 7% of all radioactive examinations, but in 2000/200 1, it accounted for 47% of the total dose of medical radiation (hart &: wall, European journal of radiology 2004; 50:285-29 1), excessive use of computed tomography, no matter how damaged in other places, will still lead to an increase in the overall medical dose. In some papers devoted to radiation dose, many factors are considered: scanning volume, patient's physique, scanning times and types, as well as required resolution and image quality. Negative reaction of contrast agent